The present invention relates to oxidative dehydrogeneration catalysts, and more particularly to oxidative dehydrogenation catalysts useful for producing diolefins from C.sub.4 to C.sub.8 mono-olefins.
The use of dehydrogenation catalysts to oxidize hydrocarbons to diolefins is known and appreciated by the prior art. For example, U.S. Pat. No. 3,927,138 relates to a process for producing diolefins from paraffins, especially the dehydrogenation of butane to butenes and butadiene, using an oxidation cataiyst comprising a ferrous metal, tin, phosphorus and an alkali metal. The catalysts may be supported on or diluted with materials such as silica, alumina, boria, etc.
U.S. Pat. No. 3,914,332 discloses a process for the oxidative dehydrogenation of butane to a mixture of butenes and butadiene using a vanadium-potassium-sulfur catalyst supported on silica, which permits the use of high space velocities.
U.S. Pat. No. 3,856,881 relates to a process for the dehydrogenation of C.sub.4 to C.sub.5 hydrocarbons to produce the corresponding dehydrogenated compounds. The dehydrogenation catalyst used contains a crystalline spinel of a phosphorus and divalent-metallic vanadium compound. Catalyst carriers such as alumina, pumice, silicon, etc., are additionally described as suitable for use in the dehydrogenation catalyst.
U.S. Pat. No. 3,789,078 discloses a dehydrogenation process and dehydrogenation catalysts useful for oxidatively dehydrogenating organic compounds such as alkenes, alkadienes, cycloalkenes, alkylpyridines and alkyl aromatics. The catalyst contains a combination of phosphorus, tin, and a Group IA or IIA metal of the Periodic Table. Substantially any phosphorus, tin, and Group IA or IIA-containing materials may be employed in the catalyst so long as at least one of the materials used contains oxygen.
U.S. Pat. No. 3,775,508 relates to an oxidative dehydrogenation process for dehydrogenating C.sub.2 to C.sub.10 alkenes, alkadienes, etc., using an oxidation catalyst containing phosphorus, tin, and a group IA or IIA metal of the Periodic Table. The catalyst is improved by including a heat-volatile activity-stimulating ammonium salt in the catalyst composition prior to the catalyst particle-forming stage.
As a rule, the prior art has avoided the use of crystalline aluminosilicate zeolites as support materials in catalysts for the oxidative dehydrogenation of hydrocarbons. Crystalline aluminosilicate zeolites are known to be useful in many hydrocarbon conversion reactions, such as cracking, hydrocracking, etc. But for hydrocarbon oxidation reactions and oxidative dehydrogenation reactions, the art has not succeeded with zeolitic catalysts.
Accordingly, it is an object of the invention to provide catalysts, and particularly zeolitic catalysts, and methods for their preparation and use, which are useful for oxidative dehydrogenation reactions, especially the conversion of C.sub.4 to C.sub.8 mono-olefins to a conjugated diene.
It is yet a further object to provide a method for producing such catalysts containing vanadium in an average valence in the range of +3.50 to +4.95.
A further object of the present invention is to provide a method for obtaining improved yields and selectivity of conjugated dienes from the corresponding mono-olefin, e.g., butadiene from butene.
These and other objects are accomplished according to the present invention by oxidizing a C.sub.4 to C.sub.8 mono-olefin to the corresponding diolefin in the presence of an alkali metal-promoted oxidative dehydrogenation catalyst comprising the oxides of vanadium and phosphorus (and optionally and preferably, tin) on a support containing a microporous crystalline silica or a crystalline zeolite having a silica-to-alumina ratio of at least 6.0.